Active energy ray curable aqueous ink composition, and inkjet recording method, ink cartridge, recording unit, and inkjet recording apparatus using the same

ABSTRACT

To provide an active energy ray curable aqueous ink composition exhibiting excellent curing performance by including a water-soluble polymerizable substance with high reactivity and a water-soluble polymerization initiator having high initiation efficiency; and an inkjet recording method, ink cartridge, recording unit, and inkjet recording apparatus all using the active energy ray curable aqueous ink composition. The active energy ray curable aqueous ink composition includes at least water; a polymerizable substance which undergoes radical polymerization by active energy rays; and a polymerization initiator which generates radicals by active energy rays, in which the polymerizable substance is a water-soluble compound having 2 to 6 (meth)acrylic groups; and the polymerization initiator is a water-soluble acylphosphine oxide compound.

This application is a continuation of International Application No.PCT/JP2005/006435, filed Mar. 25, 2005, which claims the benefit ofJapanese Patent Application No. 2004-092371, filed Mar. 26, 2004.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a novel active energy ray curableaqueous ink composition exhibiting excellent curing performance byincluding a water-soluble polymerizable substance having high reactivitywhich undergoes radical polymerization by active energy rays and awater-soluble polymerization initiator having high initiation efficiencywhich generates radicals by active energy rays, and to an inkjetrecording method, ink cartridge, recording unit, and inkjet recordingapparatus using the active energy ray curable aqueous ink composition.

2. Related Background Art

Active energy ray curable aqueous ink can be preferably used for imageprinting, pretreatment for providing printability to a recording medium,after treatment for protection or decoration of the printed image, andthe like. Further, the active energy ray curable aqueous ink technologyhas numerous excellent features and possibilities in that the ink cankeep up with tightening of regulations on VOC, can be applied to a highdensity inkjet recording head through reduction of ink viscosity, etc.

A polymerizable substance which undergoes radical polymerization byactive energy rays (hereinafter, referred to as polymerizable substance)and a polymerization initiator which generates radicals by active energyrays (hereinafter, referred to as polymerization initiator), which arebasic components of the active energy ray curable aqueous ink, each hadbeen generally prepared in a form of emulsion in a water-based solventinitially. However, the polymerizable substance and the polymerizationinitiator themselves must be water-soluble or hydrophilic to preparehomogenous ink from the viewpoints of film formation properties of theink during polymerization, discharge stability thereof applied to aninkjet system, and the like.

A first example of a water-soluble or hydrophilic polymerizablesubstance in practical use is a compound having an acidic group, and a(meth)acryloyl group or vinyl group in a molecule such as an ester ofsuccinic anhydride and 2-hydroxyethyl(meth)acrylate, an ester ofphthalic anhydride and 2-hydroxyethyl(meth)acrylate, or vinylnaphthalenesulfonic acid.

A second example of the water-soluble or hydrophilic polymerizablesubstance in practical use is a (meth)acrylic ester of a polyhydricalcohol provided with hydrophilicity by a polyethylene oxide chain suchas tetraethylene glycol di(meth)acrylate, which is an industriallyproduced compound having two or more polymerizable functional groups.

Another example thereof is a (meth)acrylic ester of a hydrophilicpolyepoxide derived from a polyhydric alcohol such as glycerin,disclosed in Japanese Patent Application Laid-Open No. 2000-117960.

Examples of a water-soluble or hydrophilic polymerization initiatorinclude: a hydroxyphenyl ketone polymerization initiator provided withwater-solubility through addition of a polyethylene oxide chain,disclosed in Japanese Patent Application Laid-Open No. 2002-186242; anda thioxanthone polymerization initiator provided with water-solubilitythrough addition of a polyethylene oxide chain or an anionic functionalgroup, disclosed in Japanese Patent Application Laid-Open No.2002-187918. Further, Japanese Patent Application Laid-Open No.S57-197289 discloses an invention relating to an acylphosphine oxidepolymerization initiator having an ionic group as a polymerizationinitiator useful for polymerization of an aqueous polymerizablecomposition such as a coating material, a printing ink, an unsaturatedpolyester molding material, or a recording material.

Further, Japanese Patent Application Laid-Open No. 2000-159621 disclosesan invention in which the water-soluble acylphosphine oxidepolymerization initiator described in Japanese Patent ApplicationLaid-Open No. S57-197289 is used as a polymerizable composition fordental use.

Active energy ray curable aqueous ink has excellent properties asdescribed above, but degradation of its curing performance throughdilution with water is pointed out in some cases. The curing performancemay be improved by increasing contents of a polymerizable substance andpolymerization initiator. However, when active energy ray curableaqueous ink is applied to an inkjet system, the ink is subjected tolimitation in viscosity or the like. As a result, the curing performancegreatly varies depending on properties of individual materials.

Active energy ray curable aqueous ink preferably applied to the inkjetsystem must include a water-soluble polymerizable substance which isformed of a material having low viscosity and favorable flowabilityapplicable to high density nozzles, which can be added to the ink in alarge amount, and which provides excellent hardness of an ink film orexcellent adhesion of the ink film on to a recording medium aftercuring. The active energy ray curable aqueous ink preferably appliedthereto must further include a water-soluble polymerization initiatorwhich highly responds to irradiation of active energy rays in a shortperiod of time, which can generate large amounts of radicals, and whichprovides excellent initiation efficiency.

The polymerizable substance in the first example has one polymerizablefunctional group in a molecule, and has a low polymerization rate and avery low degree of crosslinking of a cured product. Thus, thepolymerization substance cannot serve as a main material for the aqueouspolymerizable composition.

The polymerizable substance of the second example is not water-solubleif a length of an ethylene oxide chain is short. In contrast, if thelength of the ethylene oxide chain is long, the polymerizable substanceis water-soluble, but has problems in that physical properties of apolymerized or cured solid are often insufficient in performance such ashardness and adhesiveness, required for paint or ink.

The polymerizable substance disclosed in Japanese Patent ApplicationLaid-Open No. 2000-117960 is highly water-soluble, has a highpolymerization rate by active energy rays, and has excellent physicalproperties of a cured product. However, the polymerizable substance hassuch a feature that it has numerous hydroxyl groups and thus has aslightly high viscosity. Thus, the polymerizable substance may be addedto ink in a small amount.

The polymerization initiator disclosed in Japanese Patent ApplicationLaid-Open No. 2002-186242 is liquid, has excellent compatibility withwater and a polymerizable substance, but provides slightly insufficientinitiation efficiency. The polymerization initiator disclosed inJapanese Patent Application Laid-Open No. 2002-187918 is the type ofhydrogen abstraction and requires a hydrogen donor such as tertiaryamine. Further, the polymerization initiator is colored and is notsuitable for clear ink. The polymerization initiator disclosed inJapanese Patent Application Laid-Open No. S57-197289 has features ofhigh quantum yield of radical generation, high reactivity of radicalshaving unpaired electrons on a phosphorus atom, and long actual life ofgenerated radicals. Thus, the polymerization initiator has highinitiation efficiency and favorable water-solubility.

The water-soluble polymerization initiator disclosed in Japanese PatentApplication Laid-Open No. S57-197289 has excellent properties asdescribed above, but an actual application thereof remains as the dentalmaterial described in Japanese Patent Application Laid-Open No.2000-159621 or the like. In this application, an irradiation time of theactive energy rays is not particularly limited, and thus the initiationefficiency is not a required item.

High initiation efficiency is effective in applications requiring highcuring rate of printing ink or the like. Such application presumably hasnot been tried because of insufficient reactivity of a water-solublepolymerizable substance combined with a polymerization initiator.

In contrast, performance of the entire aqueous polymerizable compositionmay significantly degrade because of insufficient initiation efficiencyof a water-soluble polymerization initiator combined with awater-soluble polymerizable substance having high reactivity.

As described above, the inventors of the present invention have foundfor the first time that a combination of two factors includingreactivity of a water-soluble polymerizable substance and initiationefficiency of a water-soluble polymerization initiator must be focusedfor ensuring sufficient curing performance as an active energy raycurable aqueous ink composition and obtaining adaptability to a widerange of applications.

SUMMARY OF THE INVENTION

Therefore, a first object of the present invention is to provide anactive energy ray curable aqueous ink composition exhibiting excellentcuring performance by including a water-soluble polymerizable substancehaving high reactivity and a water-soluble polymerization initiatorhaving high initiation efficiency.

A second object of the present invention is to provide an active energyray curable aqueous ink composition exhibiting excellent curingperformance and suitably applied to an inkjet system by including awater-soluble polymerizable substance having high reactivity and awater-soluble polymerization initiator having high initiationefficiency.

A third object of the present invention is to provide an inkjetrecording method, ink cartridge, recording unit, and inkjet recordingapparatus all using the active energy ray curable aqueous inkcomposition.

The above-described objects can be attained by the present invention.That is, the present invention relates to an active energy ray curableaqueous ink composition including at least water, a polymerizablesubstance which undergoes radical polymerization by active energy rays,and a polymerization initiator which generates radicals by active energyrays, characterized in that: the polymerizable substance is awater-soluble compound having 2 to 6 (meth)acrylic groups, and thepolymerization initiator is a water-soluble acylphosphine oxidecompound.

Further, the present invention relates to an inkjet recording methodincluding the steps of: discharging the aqueous ink composition to arecording medium by using an inkjet system; and curing the aqueous inkcomposition by irradiating the recording medium provided with theaqueous ink composition with active energy rays.

Further, the present invention relates to an ink cartridge including anink container containing the aqueous ink composition.

Further, the present invention relates to a recording unit including anink container containing the aqueous ink composition, and a recordinghead for discharging the aqueous ink composition.

Further, the present invention relates to an inkjet recording apparatusincluding means for providing the aqueous ink composition to a recordingmedium, and means for irradiating the aqueous ink composition providedon the recording medium with active energy rays.

An aspect of the present invention provides an active energy ray curableaqueous ink composition exhibiting excellent curing performance andsuitably used for an inkjet system. That is, the aspect of the presentinvention provides an active energy ray curable aqueous ink compositionexhibiting excellent curing performance by including a water-solublepolyfunctional acrylic compound having high reactivity as apolymerizable substance, and a water-soluble acylphosphine oxidecompound having high initiation efficiency as a polymerizationinitiator.

Another aspect of the present invention provides an active energy raycurable aqueous ink composition exhibiting excellent curing performanceand suitably used for an inkjet system, and a recording method includingthe steps of providing the active energy ray curable aqueous inkcomposition to a recording medium by using an inkjet system, and curingthe aqueous ink composition by immediately irradiating the recordingmedium with active energy rays. Thus, an image having high quality andhigh fastness can be formed on a recording medium with high absorbencysuch as plain paper.

BRIEF DESCRIPTION OF THE DRAWING

FIGURE is a schematic front view of a printer preferably used for aninkjet recording method according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the present invention will be described more specificallyby way of preferred embodiments. The inventors of the present inventionhave conducted studies in view of the above-described objects, and haveprepared an active energy ray curable aqueous ink composition, includingat least water, a polymerizable substance which undergoes radicalpolymerization by active energy rays, and a polymerization initiatorwhich generates radicals by active energy rays, in which thepolymerizable substance is a water-soluble compound having 2 to 6(meth)acrylic groups, and the polymerization initiator is awater-soluble acylphosphine oxide compound. The inventors of the presentinvention have evaluated the active energy ray curable aqueous inkcomposition through irradiation of active energy rays. As a result, theinventors have found that the active energy ray curable aqueous inkcomposition having such a constitution exhibits excellent curingperformance, to thereby complete the present invention.

That is, the active ray aqueous ink composition of the present inventionpresumably exhibits excellent curing performance although it is dilutedwith water by including both a water-soluble polyfunctional acryliccompound as a polymerizable substance having excellent reactivitydepending on the number of functional groups, small acrylic equivalentweight, or a hydrogen bond formed between hydroxyl groups; and awater-soluble acylphosphine oxide compound as a polymerization initiatorhaving high initiation efficiency depending on high quantum yield ofradical generation, high reactivity of radicals having unpairedelectrons on a phosphorus atom, or long actual life of generatedradicals.

Hereinafter, description is given of functions and effects of printingby using an aqueous inkjet system, which is a main application of theactive energy ray curable aqueous ink composition of the presentinvention.

Examples of the active energy rays that can be used in the presentinvention include ultraviolet light and electron beams. Hereinafter, aUV-curable aqueous ink composition which is cured by using particularlypreferable ultraviolet light may be used as a typical example indescriptions, but the descriptions do not imply that the active energyrays are limited to ultraviolet light.

Main purposes for use of the active energy ray curable aqueous inkcomposition of the present invention for image formation by using anaqueous inkjet system are those described below, for example.

1) To enhance drying properties of ink, and allow improvement inprinting speed.

2) To use a water-soluble polymerizable substance as a binder for acoloring material, and allow formation of an image with excellentrub-off resistance on various recording media.

3) To reduce scattering of light by pigment particles, and allowformation of a transparent ink layer.

4) To expand a range of color reproduction of process colors, and allowformation of an image with high density, excellent chroma, and excellentbrightness.

5) To allow protection of a coloring material from active light, and gascomponents and moisture in air.

The active energy ray curable aqueous ink composition of the presentinvention exhibits a significant effect of improving such properties aschroma of a coloring material and rub-off resistance of a recordingmedium having ink absorbency such as plain paper, for which suchproperties are hardly improved. Further, the active energy ray curableaqueous ink composition of the present invention allows printing of anonabsorbent recording medium with an aqueous ink.

It is necessary that the active energy ray curable aqueous inkcomposition of the present invention have a viscosity within anappropriate range to be used for an inkjet system. In addition it ispreferable that the ink composition has an optimized surface tension forformation of a high quality, high density image particularly on plainpaper from the viewpoint of a balance between curing and permeation.

Next, detailed description is given of components of the active energyray curable aqueous ink composition of the present invention havingexcellent functions and effects as described above, and an inkjetrecording method using the active energy ray curable aqueous inkcomposition.

(Active Energy Ray Curable Aqueous Ink Composition)

The active energy ray curable aqueous ink composition of the presentinvention includes at least water, a water-soluble compound having 2 to6 (meth)acrylic groups as a polymerizable substance which undergoesradical polymerization by active energy rays, and a water-solubleacylphosphine oxide compound as a polymerization initiator whichgenerates radicals by active energy rays. The active energy ray curableaqueous ink composition may further include an aqueous coloringmaterial, other water-soluble polymerization initiators, an aqueoussolvent, arbitrary additives, and the like.

(Water-Soluble Polymerizable Substance)

Description is given of a polymerizable substance preferably used in thepresent invention. The polymerizable substance is a water-solublecompound having 2 to 6 (meth)acrylic groups, preferably a water-soluble(meth)acrylate compound represented by the following general formula(1).

(In the general formula (1), Z represents a residue of a polyol or anepoxy ester of a polyol having (x1+y1) hydroxyl groups; x1 represents aninteger of 0 to 2; y1 represents an integer of 2 to 6; and x1+y1represents an integer of 2 to 6. R₁ each independently represent ahydrogen atom or a methyl group. R₂ each independently represent anatomic group A represented by the following formula.

(In the formula: R₃ each independently represent a hydrogen atom or amethyl group; m1 represents an integer of 1 to 5; and n1 represents 0 or1.))

The water-soluble polymerizable substance preferably used in the presentinvention has preferably 2 to 6 polymerizable functional groups, morepreferably 3 to 6 polymerizable functional groups. A larger number ofpolymerizable functional groups further improve a curing rate. However,the molecule of the polymerizable substance becomes a giant one, and theviscosity of the ink composition increases. In this case, an additionamount of the polymerizable substance is limited, and much improvementin curing performance of the active energy ray curable aqueous inkcomposition used for an inkjet system cannot be expected, in particular.It should be taken into consideration that a high curing rate suppressesdiffusion of molecules at an early stage of polymerization, andunreacted polymerizable functional groups are apt to remain. Incontrast, a water-soluble polymerizable substance having onepolymerizable functional group is not preferable because of a low curingrate and a significantly low degree of crosslinking. The polymerizablefunctional group preferably includes a (meth)acrylic group, morepreferably an acrylic group from the viewpoint of curing performance.

Further, the polymerizable substance has preferably 1 to 5, morepreferably 1 to 3 ethylene oxide chains or propylene oxide chains forproviding water-solubility thereto. Long ethylene oxide chains orpropylene oxide chains provide water-solubility, but providesinsufficient hardness of a film, poor adhesiveness of the film onto arecording medium after curing, and the like. The polymerizable substancepreferably further has a glycidyl group for attaining bothwater-solubility and curing performance. A hydrogen bond formed betweenhydroxyl groups of the glycidyl group may increase the viscosity of theink composition to limit its addition amount. However, curingperformance of the active energy ray curable aqueous ink composition issufficiently ensured by combination of the polymerizable substance withthe water-soluble polymerization initiator of the present invention withexcellent initiation efficiency.

The polyol residue represented by Z is one obtained by removing two ormore hydroxyl groups from a polyol. Examples of preferable polyolsinclude ethylene glycol, diethylene glycol, triethylene glycol,polyethylene glycol, propylene glycol, dipropylene glycol, tripropyleneglycol, polypropylene glycol, 1,3-propanediol, 1,2-butanediol,1,3-butanediol, 1,4-butanediol, 2,3-butanediol, 1,5-pentanediol,1,4-pentanediol, 2,4-pentanediol, 3-methyl-1,5-pentanediol,2-methyl-2,4-pentanediol, 1,5-hexanediol, 1,6-hexanediol,2,5-hexanediol, glycerin, 1,2,4-butanetriol, 1,2,6-hexanetriol,1,2,5-pentanetriol, thiodiglycol, trimethylolpropane,ditrimethylolpropane, trimethylolethane, ditrimethylolethane, neopentylglycol, pentaerythritol, dipentaerythritol, and a condensate thereof.Examples of other polyols include, but are not limited to, low molecularweight vinyl alcohols; monosaccharides such as triose, tetrose(erythritol or threitol), pentose (ribitol, arabinitol, or xylitol),hexose (allitol, altritol, glucitol, mannitol, iditol, galactitol, orinositol), heptose, octose, nonose and decose, and deoxy sugars thereof;aldonic acid and aldaric acid derivatives.

Of those, examples of particularly preferable polyols include glycerin,1,2,4-butanetriol, 1,2,6-hexanetriol, 1,2,5-pentanetriol,trimethylolpropane, trimethylolethane, neopentyl glycol, andpentaerythritol.

Specific examples of a particularly preferable water-solublepolymerizable substance include water-soluble compounds having thefollowing structures, but the water-soluble polymerizable substance usedin the present invention is not limited thereto.

Of those, Exemplified Compounds 2, 3, and 4 are particularly preferable.

(Water-Soluble Polymerization Initiator)

Description is given of a polymerization initiator preferably used inthe present invention. Examples of the polymerization initiator includea water-soluble acylphosphine oxide compound, preferably one of anonionic water-soluble acylphosphine oxide compound and an anionicwater-soluble acylphosphine oxide compound represented by the followinggeneral formula (2).

(In the general formula (2), R₄ represents an alkyl group or a phenylgroup; R₅ represents an alkyloxy group or a phenyl group; and R₆represents an atomic group B represented by the following formula.

(In the formula, R₇ represents —[CH₂]_(x2)— (x2 represents 0 or 1) or aphenylene group; m2 represents an integer of 0 to 10; and n2 represents0 or 1. R₈ represents a hydrogen atom, a sulfonic group, a carboxylgroup, a hydroxyl group, or a salt thereof.))

The alkyl group or phenyl group represented by R₄ may have asubstituent. Examples of the substituent include a halogen, a loweralkyl group having 1 to 5 carbon atoms, a lower alkyloxy group having 1to 5 carbon atoms, the above atomic group B, a sulfonic group, acarboxyl group, a hydroxyl group, and salts of a sulfonic group, acarboxyl group, and a hydroxyl group (—SO₃M, —CO₂M, and —OM). M eachindependently represent a hydrogen atom, an alkali metal, an alkaliearth metal, or ammonium represented by HNR₉R₁₀R₁₁ (R₉, R₁₀; and R₁₁each independently represent a hydrogen atom, a lower alkyl group having1 to 5 carbon atoms, a monohydroxyl-substituted lower alkyl group having1 to 5 carbon atoms, or a phenyl group).

R₄ is particularly preferably a phenyl group having as a substituent alower alkyl group having 1 to 5 carbon atoms.

The phenylene group represented by R₇ may have a substituent. Examplesof the substituent include a halogen, a lower alkyl group having 1 to 5carbon atoms, a lower alkyloxy group having 1 to 5 carbon atoms, asulfonic group, a carboxyl group, a hydroxyl group, and salts of asulfonic group, a carboxyl group, and a hydroxyl group (—SO₃M, —CO₂M,and —OM). M each independently represent a hydrogen atom, an alkalimetal, an alkali earth metal, or ammonium represented by HNR₉R₁₀R₁₁ (R₉,R₁₀, and R₁₁ each independently represent a hydrogen atom, a lower alkylgroup having 1 to 5 carbon atoms, a monohydroxyl-substituted lower alkylgroup having 1 to 5 carbon atoms, or a phenyl group).

Examples of the salt represented by R₈ include —SO₃M, —CO₂M, and —OM. Meach independently represent a hydrogen atom, an alkali metal, an alkaliearth metal, or ammonium represented by HNR₉R₁₀R₁₁ (R₉, R₁₀ and R₁₁ eachindependently represent a hydrogen atom, a lower alkyl group having 1 to5 carbon atoms, a monohydroxyl-substituted lower alkyl group having 1 to5 carbon atoms, or a phenyl group).

The acylphosphine oxide polymerization initiator absorbs light of alonger wavelength region than that of a polymerization initiatorgenerally used and has excellent light transmittance. Further, theacylphosphine oxide polymerization initiator generates acyl radicals,and phosphinoyl radicals with high reactivity by having unpairedelectrons on phosphorus atoms through cleavage by irradiation of activeenergy rays, to thereby have high initiation efficiency. The cleavagecauses reduction in absorption or color fading, and the acylphosphineoxide polymerization initiator can suppress coloring and be used forcuring of a thick film.

The acylphosphine oxide compound preferably used in the presentinvention provides water-solubility with a nonionic functional and/oranionic functional group. Examples of the nonionic functional groupinclude but are not limited to a polyethylene oxide chain and apolypropylene oxide chain. Examples of the anionic functional groupinclude but are not limited to a sulfonic group, a carboxyl group, ahydroxyl group, and salts thereof (—SO₃M, —CO₂M, and —OM (wherein, Mpreferably represents a hydrogen atom; an alkali metal such as lithium,sodium, or potassium; or ammonium such as ammonium,dimethylethanolammonium, methyldiethanolammonium, triethanolammonium, oranilinium).

Specific examples of a particularly preferable water-solublepolymerization initiator include water-soluble compounds having thefollowing respective structures, but the water-soluble polymerizationinitiator used in the present invention is not limited thereto.

(Coloring Material)

In the present invention, the addition of a coloring material provides acurable and colored active energy ray curable aqueous ink composition,and the application thereof to a multicolor inkjet system allowsmulticolor image formation. A basic factor required for a coloringmaterial that can be preferably used for the active energy ray curableaqueous ink composition of the present invention is that compatibilitybetween the water-soluble polymerizable substance and the water-solublepolymerization initiator, which is essential for providing active energyray curable ink, be satisfied. To be specific, an aqueous pigmentdispersion or aqueous dye having an anionic dissociation group may bepreferably used.

(Anionic Aqueous Pigment Dispersion)

An example of a black pigment includes carbon black such as carbon blackproduced through a furnace process or a channel process. Carbon blackthat can be preferably used has such properties as a primary particlesize of 15 to 40 μm; a specific surface area of 50 to 300 m²/g through aBET method; a DBP oil absorption of 40 to 150 ml/100 g; a volatilecomponent of 0.5 to 10%; and a pH of 2 to 9. Examples of commerciallyavailable carbon black having such properties and that can be preferablyused include: No. 2300, No. 900, MCF 88, No. 33, No. 40, No. 52, MA7,MA8, and No. 2200B (all available from Mitsubishi Chemical Corporation);RAVEN 1255 (available from Columbian Chemicals Co.); REGAL 400R, REGAL330R, REGAL 660R, and MOGUL L (all available from Cabot Corporation);Color Black FW1, Color Black FW18, Color Black S170, Color Black S150,Printex 35, and Printex U (all available from Degussa Corporation).

Examples of a yellow pigment include Pigment Yellow 1, Pigment Yellow 2,Pigment Yellow 3, Pigment Yellow 12, Pigment Yellow 13, Pigment Yellow14, Pigment Yellow 16, Pigment Yellow 17, Pigment Yellow 55, PigmentYellow 73, Pigment Yellow 74, Pigment Yellow 75, Pigment Yellow 83,Pigment Yellow 93, Pigment Yellow 95, Pigment Yellow 97, Pigment Yellow98, Pigment Yellow 109, Pigment Yellow 110, Pigment Yellow 114, PigmentYellow 128, Pigment Yellow 138, Pigment Yellow 139, Pigment Yellow 150,Pigment Yellow 151, Pigment Yellow 154, Pigment Yellow 155, and PigmentYellow 180.

Examples of a magenta pigment include Pigment Red 5, Pigment Red 7,Pigment Red 12, Pigment Red 48 (Ca), Pigment Red 48 (Mn), Pigment Red57:1, Pigment Red 57 (Sr), Pigment Red 57:2, Pigment Red 122, PigmentRed 123, Pigment Red 168, Pigment Red 184, Pigment Red 202, and PigmentRed 238.

Examples of a cyan pigment include Pigment Blue 1, Pigment Blue 2,Pigment Blue 3, Pigment Blue 16, Pigment Blue 22, Pigment Blue 60,Pigment Blue 15:2, Pigment Blue 15:3, Vat Blue 4, and Vat Blue 60.

The pigment particles preferably have an average particle size of about25 nm to 200 nm. The range varies depending on the use of a printedproduct but is sufficiently smaller than wavelengths of visible lightand provides a printed product with sufficient transparency with littlescattering of light.

(Anionic Aqueous Dye)

The use of a dye as a coloring material is different from the use of thepigment as a coloring material. The dye is hardly used without colorfading by irradiation of active energy rays, and thus causes some colorfading. For this reason, when a dye is used as a coloring material forink, a so-called azo-containing dye forming a complex of metal ions ispreferably used to suppress color fading. However, if a color loss levelis not taken into consideration, a general water-soluble dye serves atleast as an ink composition.

Examples of a black dye include monoazo or diazo complexes havingpolyvalent metals such as Cr, Cu, Mn, Al, Zn, and Fe coordinatedthereon; and noncomplex azo black dyes. Examples thereof that can bepreferably used include Direct Black 17, Direct Black 19, Direct Black51, Direct Black 154, Direct Black 174, and Direct Black 195.

Examples of a yellow dye include Acid Yellow 11, Acid Yellow 17, AcidYellow 23, Acid Yellow 25, Acid Yellow 29, Acid Yellow 42, Acid Yellow49, Acid Yellow 61, Acid Yellow 71, Direct Yellow 12, Direct Yellow 24,Direct Yellow 26, Direct Yellow 44, Direct Yellow 86, Direct Yellow 87,Direct Yellow 98, Direct Yellow 100, Direct Yellow 130, Direct Yellow132, and Direct Yellow 142.

Examples of a magenta dye include Acid Red 1, Acid Red 6, Acid Red 8,Acid Red 32, Acid Red 35, Acid Red 37, Acid Red 51, Acid Red 52, AcidRed 80, Acid Red 85, Acid Red 87, Acid Red 92, Acid Red 94, Acid Red115, Acid Red 180, Acid Red 254, Acid Red 256, Acid Red 289, Acid Red315, Acid Red 317, Direct Red 1, Direct Red 4, Direct Red 13, Direct Red17, Direct Red 23, Direct Red 28, Direct Red 31, Direct Red 62, DirectRed 79, Direct Red 81, Direct Red 83, Direct Red 89, Direct Red 227,Direct Red 240, Direct Red 242, and Direct Red 243.

Examples of a cyan dye include Acid Blue 9, Acid Blue 22, Acid Blue 40,Acid Blue 59, Acid Blue 93, Acid Blue 102, Acid Blue 104, Acid Blue 113,Acid Blue 117, Acid Blue 120, Acid Blue 167, Acid Blue 229, Acid Blue234, Acid Blue 254, Direct Blue 6, Direct Blue 22, Direct Blue 25,Direct Blue 71, Direct Blue 78, Direct Blue 86, Direct Blue 90, DirectBlue 106, and Direct Blue 199.

A preferable concentration of the dye in ink is 0.1 to 10% by mass withrespect to the total amount of the ink. Ink having a low dyeconcentration is preferably used for, for example, a light-colored inkof the density modulation ink.

(Other Water-Soluble Polymerization Initiators)

In the present invention, other water-soluble polymerization initiatorsmay be used in combination with the water-soluble acylphosphine oxidepolymerization initiator. Preferable examples of the other water-solublepolymerization initiators include at least one compound selected fromthe group consisting of water-soluble compounds represented by thefollowing respective general formulae (3) to (5).

(In the general formula (3), m3 represents an integer of 1 or more; n3represents an integer of 0 or more; and m3+n3 represents an integer of 1to 8.)

(In the general formula (4), R₁₂ and R₁₃ each independently represent ahydrogen atom or an alkyl group; and m4 represents an integer of 5 to10.)

(In the general formula (5), R₁₂ and R₁₃ each independently represent ahydrogen atom or an alkyl group; R₁₄ represents —[CH₂]_(x3)— (x3represents 0 or 1), —O—[CH₂]_(y3)— (y3 represents 1 or 2), or asubstituted or unsubstituted phenylene group; and M represents ahydrogen atom or an alkali metal.)

An absorption region of the acylphosphine oxide polymerization initiatorextends to a long wavelength region of 400 nm or more, and light of thelong wavelength region easily penetrates into the inner part, thereforeacylphosphine oxide polymerization initiator is superior to provideinner curing performance in a system containing a pigment. However, theacylphosphine oxide polymerization initiator (in particular, phosphinoylradicals generated on phosphorus atoms) is easily affected by oxygeninhibition and may exhibit insufficient surface curing performance. Incontrast, other polymerization initiators each having an absorptionregion in a short wavelength region may be added to aid curing of asurface layer.

The use of the thioxanthone polymerization initiator represented by thegeneral formula (4) or (5) preferably involves addition oftriethanolamine or the like as a hydrogen donor.

Specific examples of preferable other water-soluble polymerizationinitiators that can be used in combination with the acylphosphine oxidepolymerization initiator include water-soluble compounds represented bythe following respective structures. However, other water-solublepolymerization initiators usable in the present invention are notlimited thereto.

(Solvent Component)

A slight amount of a solvent component is preferably added to the activeenergy ray curable aqueous ink composition of the present invention. Thesolvent component is added for conventional purposes such as provisionof nonvolatile properties to the ink, reduction in viscosity of the ink,provision of wettability of the ink on a recording medium, andsuppression of fixing of the ink to nozzles by maintaining dispersionstability of a pigment. In addition, the solvent component is added forthe purpose of alleviating shrinkage on curing in a system with a highpolymerization rate using a polymerizable substance having highreactivity and a high crosslinking density and a polymerizationinitiator having high initiation efficiency such as the active energycuring aqueous ink composition of the present invention. Further, whenthe polymerization advances to some extent in such system, diffusion ofthe polymerizable substance or the polymerization initiator issuppressed to inhibit increase of a final curing rate. The solventcomponent serves as an effective countermeasure thereto as well.

A printed mater obtained by using an inkjet system is often picked upand enjoyed by unspecified number of people. Therefore, it is importantthat a cured film having sufficient strength and flexibility be formedto provide fastness such that no peeling takes place from a recordingmedium.

The solvent is preferably selected from glycerin, ethylene glycol,diethylene glycol, and a mixture thereof having high inkjetrecordability. The solvent is generally included in an amount of 0.5 to5% by mass with respect to the total amount of the ink composition.

(Reactive Diluent Composition)

Water is mainly used as a solvent for diluting the water-solublepolymerizable substance or water-soluble polymerization initiator of thepresent invention, but a water-soluble, polymerizable, and low viscositymonomer may be used as a solvent. An advantage in the use of suchsubstance and not a normal solvent lies in that the substance does notremain as a plasticizer in a solid after the curing by active energyrays, to thereby reduce its effect on the physical properties of thesolid.

Examples of a reactive diluent solvent component selected for theabove-described purpose include acryloylmorpholine,N-vinyl-2-pyrrolidone, N-vinylformamide, acrylamide,methylenebisacrylamide, monoacrylates of monosaccharides, a monoacrylicester of an oligoethylene oxide, and a monoacrylic ester of a dibasicacid.

(Additives)

In the present invention, the active energy ray curable aqueous inkcomposition may further include arbitrary additives in addition to theabove-described substances. Examples of the additives include a pHadjustor, a leveling agent, a viscosity modifier, an antioxidant, ahindered amine light stabilizer (HALS), an antiseptic, and a fungicide.The additives are generally used in an amount of 0.1 to 5% by mass withrespect to the total amount of the ink composition.

(Preparation of Clear Ink)

The active energy ray curable aqueous ink composition of the presentinvention may be transparent without the coloring material as describedabove, to be used as an undercoat for providing image printability to arecording medium, an overcoat for protecting a surface of an imageformed with conventional ink and for further providing decoration orgloss thereto, or the like. The clear ink may contain as necessary acolorless pigment not used for coloring, fine particles, or the likedispersed therein. Such addition of the clear ink in an undercoat or anovercoat improves properties of the printed product such as imagequality, fastness, and handling property.

The active energy ray curable aqueous ink composition of the presentinvention used for such clear ink preferably contains 10 to 70% by massof the water-soluble polymerizable substance with respect to the totalamount of the ink. Further, the clear ink is preferably prepared suchthat the clear ink contains 1 to 10 parts by mass of the polymerizationinitiator with respect to 100 parts by mass of the polymerizablesubstance, and at least 0.5 part by mass of the polymerization initiatorwith respect to 100 parts by mass of the ink composition.

(Components and Physical Properties of Ink Composition ContainingColoring Material)

The active energy ray curable aqueous ink composition of the presentinvention using a coloring material such as a pigment preferably has aconcentration of a pure pigment of about 0.3 to 10% by mass in an inkwith respect to the total amount of the ink. Coloring power of thepigment varies depending on a dispersed state of the pigment particles,but the ink composition may be used as a light-colored ink when thepigment concentration falls within a range of about 0.3 to 1%. The inkcomposition having the pigment concentration out of the above range isused for general colored ink.

Regarding contents of other components, a water content is preferably 40to 90% by mass, particularly preferably 60 to 75% by mass with respectto the total amount of the ink. Further, the content of thepolymerizable substance in the ink is preferably 1% to 30% by mass,particularly preferably 5 to 20% by mass with respect to the totalamount of the ink. The content of the polymerization initiator variesdepending on the content of the polymerizable substance, but ispreferably 0.1 to 7% by mass, particularly preferably 0.3 to 5% by masswith respect to the total amount of the ink.

In a case where the active energy ray curable aqueous ink composition ofthe present invention is applied to an inkjet system, nonlinearity isnot found in a broad range of viscosity and the upper limit thereof is15 mPa·s. In the case where the active energy ray curable aqueous inkcomposition is applied to an inkjet system provided with fine,high-density, and high-driving frequency nozzles, the upper limit of theviscosity is preferably 10 mPa·s.

The active energy ray curable aqueous ink composition preferably has asurface tension of 35 mN/m (dyne/cm) or more in view of printing onplain paper. For a normal aqueous inkjet ink, the surface tension isadjusted to a low value of about 30 mN/m, to allow permeation of the inkwithin a short period of time. Thus, a bleeding phenomenon is suppressedwith reduction of image density. Meanwhile, flow of the active energyray curable aqueous ink composition of the present invention can besuppressed by curing. Thus, the surface tension is increased toaccumulate ink droplets on a surface layer of a recording medium as muchas possible, to thereby satisfy bleeding properties and image density.

For ensuring image density, the ink droplets are required to be somewhatwet on a recording medium upon irradiation of active energy rays. Thus,the upper limit of the surface tension is preferably about 50 mN/m.

(InkJet Recording Method)

Next, description is given of an inkjet recording method for applyingthe above-described active energy ray curable aqueous ink compositiononto a recording medium by using an inkjet system, and irradiating themedium with an active energy ray to cure the ink composition.

(Printer System)

The above-described active energy ray curable aqueous ink compositionaccording to the present invention is suitably used for an inkjetdischarge type recording head, and is effectively applicable to an inkcontainer containing the ink composition or ink to be filled in thecontainer. In particular, the present invention produces beneficialeffects when applied to a bubble-jet recording head and apparatus out ofvarious inkjet recording heads and apparatuses.

As a typical arrangement or principle thereof, it is preferable toconform to the basic principle disclosed in, for example, U.S. Pat. No.4,723,129 or U.S. Pat. No. 4,740,796. This system is applicable to botha so-called on-demand type and continuous type. The system isparticularly effective in the on-demand type because at least onedriving signal corresponding to recording information and inducing arapid temperature rise beyond a film boiling point is applied to anelectrothermal transducer arranged in a position corresponding to asheet or liquid channel that retains ink, thereby causing theelectrothermal transducer to generate thermal energy to cause filmboiling on the heat acting surface of a recording head, and consequentlyforming bubbles in the ink in a one-to-one correspondence with thedriving signals. Along with the growth and shrinkage of the bubbles, theink is discharged from a discharge orifice to form at least one droplet.This driving signal is more preferably a pulsed signal because thebubble can promptly and appropriately grow and shrink, making itpossible to discharge the ink with high responsiveness. This pulseddriving signal is preferably a signal described in U.S. Pat. No.4,463,359 or U.S. Pat. No. 4,345,262. Note that printing can be moreefficiently performed under the conditions described in U.S. Pat. No.4,313,124 that discloses the invention relating to the rate oftemperature rise on the heat acting surface.

Regarding the arrangement of a recording head, it is possible to use, inaddition to the combined arrangement (linear liquid channel orright-angled liquid channel) of the discharge orifices, liquid channels,and electrothermal transducers as disclosed in the specificationsdescribed above, arrangements in which the heat acting surface is in abent region as disclosed in U.S. Pat. No. 4,558,333 and U.S. Pat. No.4,459,600. Besides, the present invention is effectively applicable to adischarge system where a recording head communicates with the air asdisclosed in Japanese Patent Nos. 2,962,880 and 3,246,949, and JapanesePatent Application Laid-Open No. H11-188870. In addition, the presentinvention is effective for the arrangement in which a common dischargeorifice is used as a discharge portion of plural electrothermaltransducers (Japanese Patent Application Laid-Open No. S59-123670,etc.). Furthermore, as a full-line type recording head having a lengthcorresponding to the width of the maximum recording medium on which arecording apparatus can conduct recording, it is possible to use eitherthe arrangement that ensures the full-line length by combining aplurality of recording heads as disclosed in the above specifications orthe arrangement where plural recording heads are integrally formed intoa single head. In either case, the present invention can enhance theabove effects more effectively.

In addition, the present invention is effective in the case of using anexchangeable chip type recording head which can be electricallyconnected to an apparatus main body and can receive ink from theapparatus main body by being mounted to the apparatus main body or acartridge type recording head, in which an ink tank is integrally set inthe recording head itself. In the present invention, it is preferable toadd recovery means for the recording head, preliminary auxiliary means,and the like as components of the recording apparatus used since thebeneficial effects of the present invention can be attained in a morestable manner. Specific examples of such means include, for therecording head, capping means, cleaning means, pressurizing or suctionmeans, and preliminary heating means using electrothermal transducers,another heating element, or a combination thereof. It is also effectivefor stable recording to set a preliminary discharge mode for dischargeindependent of printing.

A recording apparatus for implementing the recording method of thepresent invention includes, as illustrated in the schematic front viewof a printer of FIGURE, an ink tank portion 1 containing the activeenergy ray curable aqueous ink composition of the present invention, ahead portion 2 for actual recording (multi-head composed of a number ofrecording heads arranged in this case), a lamp portion 3 for applying UVlight for curing the composition, a driving portion 4 for driving thehead portion and lamp portion, and a sheet delivery portion 5 forfeeding a recording medium on which recording is conducted. Note that awiping portion, capping portion, sheet-feeding portion, and a drivingmotor portion (not shown) are provided aside from the above components.

In FIGURE, nozzle portions for discharging the active energy ray curableaqueous ink composition are symmetrically arranged for each color in thehead portion 2. The head portion 2 and the lamp portion 3 are integrallymoved laterally. Immediately after being applied to the recordingmedium, the active energy ray curable aqueous ink composition isirradiated with UV light (the lamps will be described in detail below).Hence, this recording apparatus enables inkjet recording methods capableof effectively suppressing bleeding of ink droplets during recording orbleeding between color inks on plain paper, and forming an imageexcelling in fastness as well.

In addition, the ink tank portion 1 includes four colors of black (Bk),cyan (C), magenta (M), and yellow (Y). However, another two colors oflight cyan (LC) and light magenta (LM) may be added thereto therebyproviding six colors in total with a view to recording (forming) animage of higher resolution. Further, black is inferior in responsivenessto other colors, so only three colors of cyan, magenta, and yellow maybe arranged since process black can be created by mixing the threecolors. In the present invention, the tank used has light-shieldingproperty.

Note that in the present invention, it is possible to appropriatelychoose a system where a lamp is placed in front of the sheet deliveryportion, a system where a recording medium is wound around a rotary drumupon sheet feeding/delivery, or a system where a drying portion isadditionally provided aside from the above recording system.

(UV Irradiation Lamp)

Hereinafter, description is given of a UV irradiation lamp suitably usedespecially for curing the active energy ray curable aqueous inkcomposition according to the present invention. The UV irradiation lampis preferably a so-called low-pressure mercury lamp such as has mercuryvapor pressure of 1 to 10 Pa when turned on, a high-pressure mercurylamp, and a mercury-arc lamp coated with a phosphor. An emissionspectrum of those mercury lamps in a UV range is 450 nm or shorter,particularly preferably 184 nm to 450 nm, which is suitable forefficiently inducing reaction of polymerizable materials in black orcolor ink. In addition, such lamps are preferred because, uponincorporating a power source to a printer, a compact one can be used.Examples of the mercury lamps that have been put into practical useinclude a metal halide lamp, a high-pressure mercury-arc lamp, anultra-high-pressure mercury-arc lamp, a xenon flash lamp, a deep UVlamp, a lamp externally exciting a mercury-arc lamp by using microwavewith no electrode, and a UV laser. An emission wavelength range includesthe above range. Thus, any lamp can be basically applied if a powersource size, an incident light intensity, and a lamp shape are met. Alight source is selected according to sensitivity of a polymerizationinitiator used as well.

The requisite UV intensity is preferably 500 to 5,000 mW/cm² in awavelength range effective for curing. A low cumulative intensityhinders the formation of an image of high quality and fastness. Further,excessively high irradiation intensity may damage the recording mediumor cause color degradation of coloring materials.

EXAMPLES

Hereinafter, the present invention will be described more specificallyby way of examples and comparative examples. However, the presentinvention is not limited to the following examples without departingfrom the scope of the present invention. It should be noted that theterms “part” and “%” refer to “part by mass” and “mass %” unlessotherwise stated.

Synthesis Example 1 Synthesis of Sodium2,4,6-trimethylbenzoylphenylphosphinate (Exemplified Compound 5)

The synthesis of Exemplified Compound 5 as a target is disclosed inJapanese Patent Application Laid-Open No. S57-197289 mentioned above.However, according to the method described in the document, an alkyliodide is produced as a by-product, so the mutagenicity of a methylcompound particularly shown as a preferable example in the document isdoubtful. The possibility that such a substance is incorporated as animpurity is not preferable in terms of safety. In view of the above, inthe present invention, an attempt was made to synthesize ExemplifiedCompound 5 by decomposing a commercially available bisacyl phosphineoxide compound under an acidic condition.

Dry pyridine containing no water was allowed to react with sulfuricanhydride in chloroform to prepare a pyridine sulfate according to anordinary method. Next, 250 g of a 15% solution ofphenyl-bis(2,4,6-trimethylbenzoyl)-phosphine oxide (trade name Irgacure819: available from Ciba Specialty Chemicals) in pyridine was prepared,and was sufficiently refluxed for dehydration. 50 g of the pyridinesulfate was added to the resultant, and the whole was boiled underreflux for 5 hours. The reaction mixture was poured onto about 500 g ofice, and a 20% aqueous solution of sodium hydroxide was gradually addedfor neutralization. The resultant suspension was filtered, and thefiltrate was distilled to remove remaining pyridine. After 1N HCl aq hadbeen gradually added to the remaining water layer for neutralization,water was distilled off, followed by drying in a vacuum. The residue wasrecrystallized from a trace amount of ethanol, and was dried in a vacuumto yield about 20 g of 2,4,6-trimethylphenylphosphinic acid. The acidwas dissolved into an excess amount of ethanol, and the solution wasgradually added to a solution of sodium hydroxide in ethanol. Theprecipitate was filtered out, washed with cool ethanol, and dried in avacuum to yield about 15 g of sodium2,4,6-trimethylbenzoylphenylphosphinate (Exemplified Compound 5).

Synthesis Example 2 Synthesis of Octaethylene Glycol2,4,6-trimethylbenzoylphenylphosphinate

20 g of phenyl-bis(2,4,6-trimethylbenzoyl)-phosphine oxide (trade nameIrgacure 819: available from Ciba Specialty Chemicals), 45 g ofpotassium permanganate, 600 ml of pyridine, and 300 ml of water weremixed, and the mixture was stirred under heating at 85° C. for 6 hours.After the reaction solution had been cooled to room temperature, theprecipitated crystal was filtered out and washed with water. Thefiltrate was concentrated, and 150 ml of 10% HCl aq was added to theresidue. The mixed solution was extracted with 200 ml of ethyl acetate 3times. The organic layer was gathered, and was washed with 200 ml of asaturated salt solution twice and dried with anhydrous magnesiumsulfate. The solution was concentrated and dried to yield 21 g of a paleyellow solid.

Next, 20 g of the solid, 600 g of polyethylene glycol 400, and 500 ml ofchloroform were mixed, and the mixture was stirred under heating at 50°C. for 1 hour. After the reaction solution had been cooled to roomtemperature, water was added for separation, and the water layer wasextracted with 1,200 ml of chloroform. The organic layer was gathered,and was washed with 1,000 ml of a saturated salt solution and dried withanhydrous magnesium sulfate. The solvent was distilled off under reducedpressure, and the residue was purified by means of silica gelchromatography to yield about 10 g of octaethylene glycol2,4,6-trimethylbenzoylphenylphosphinate (Exemplified Compound 6).

Examples 1 to 10

The respective components shown in Table 1 below were mixed andsufficiently stirred for dissolution. After that, the solution wasfiltered through a 0.50-μm filter under pressure to prepare each of theinks of Examples 1 to 10. In addition, the pH of each ink was adjustedto have a final value of 8.5 by means of an aqueous solution of sodiumhydroxide.

Comparative Examples 1 and 2

The respective components shown in Table 1 below were mixed andsufficiently stirred for dissolution. After that, the solution wasfiltered through a 0.50-μm filter under pressure to prepare each of theinks of Comparative Examples 1 and 2. In addition, the pH of each inkwas adjusted to have a final value of 8.5 by means of an aqueoussolution of sodium hydroxide.

Exemplified Compound 8 described above alone was used as a water-solublepolymerization initiator of Comparative Example 1. Exemplified Compound11 shown below was used as a water-soluble polymerizable substance ofComparative Example 2.

TABLE 1 Active energy ray curable ink aqueous compositions of examplesand comparative examples Loading (wt %) Example 1 Example 2 Example 3Example 4 Example 5 Example 6 Water-soluble Exemplified Compound 1 15polymerizable Exemplified Compound 2 15 15 15 substance ExemplifiedCompound 3 15 Exemplified Compound 4 15 Exemplified Compound 11Water-soluble Exemplified Compound 5 2 2 2 2 polymerization ExemplifiedCompound 6 2 initiator Exemplified Compound 7 2 Exemplified Compound 8Exemplified Compound 9 Hydrogen donor Triethanolamine Solvent GlycerinEthylene glycol Colorant Pigment Blue 15:3 40 40 40 40 40 40 Pigmentdispersion (Pigment content 10%) Reactive Acryloylmorpholine 5 5 5 5 5 5diluent pH adjustor 0.2N NaOH Aqueous Amount at which pH of ink becomes8.5 solution Water Ion-exchanged water 38 38 38 38 38 38 Surface tension(mN/m)/25° C. 41 43 43 44 43 42 Viscosity (mPa · s)/25° C. 2.8 3.3 5.08.9 3.6 3.5 Loading (wt %) Comparative Comparative Example 7 Example 8Example 9 Example 10 Example 1 Example 2 Water-soluble ExemplifiedCompound 1 polymerizable Exemplified Compound 2 15 15 15 15 15 substanceExemplified Compound 3 Exemplified Compound 4 Exemplified Compound 11 15Water-soluble Exemplified Compound 5 1.5 1.5 2 2 2 polymerizationExemplified Compound 6 initiator Exemplified Compound 7 ExemplifiedCompound 8 0.5 2 Exemplified Compound 9 0.5 Hydrogen donorTriethanolamine 0.5 Solvent Glycerin 2 Ethylene glycol 2 ColorantPigment Blue 15:3 40 40 40 40 40 40 Pigment dispersion (Pigment content10%) Reactive Acryloylmorpholine 5 5 5 5 5 5 diluent pH adjustor 0.2NNaOH Aqueous Amount at which pH of ink becomes 8.5 solution WaterIon-exchanged water 38 37.5 36 36 38 38 Surface tension (mN/m)/25° C. 4242 41 41 42 41 Viscosity (mPa · s)/25° C. 3.5 3.4 3.5 3.6 3.7 2.3

(Evaluation of Curing Performance)

The active energy ray curable aqueous ink composition according to thepresent invention exhibits high curing performance even in a system witha pigment added and can correspond to high-speed printing on plain paperbecause the ink composition contains a water-soluble polyfunctionalacrylic compound having high reactivity as a polymerizable substance,and a water-soluble acyl phosphine oxide compound having high initiationefficiency as a polymerization initiator. The curing performance wasevaluated by means of modified a photo-differential scanning calorimetry(photo-DSC) device and a bubble jet printer equipped with a UVirradiation lamp.

(1) Photo-DSC Measurement

About 1.5 mg of each of the active energy ray curable aqueous inkcompositions of Examples 1 to 10 according to the present invention andComparative Examples 1 and 2 were weighed in an aluminum pan, and thealuminum pan was mounted on the photo-DSC device (manufactured byMettler-Toledo K. K.) together with an empty aluminum pan for reference.After having been left standing under the atmosphere at 25° C. for 10minutes, the pans were irradiated with ultraviolet light having anintensity of 400 mW/cm² at 365 nm for 0.1 second. The resultant DSCcurve was normalized to a value per unit mass, and the gradient ofcalorific power/time (W/g·s) from the rise-up of an exothermic peak tothe apex was measured and evaluated. The gradient is intended forcomparing the responsiveness of curing with respect to a constantirradiation energy. The larger the gradient value is, the more excellentthe curing performance is. Table 2 summarizes the results of theevaluation (Values each show a relative value when the value of Example2 is set to be 1.0.).

(2) Printing Evaluation 1

Each of the active energy ray curable aqueous ink compositions ofExamples 1 to 10 according to the present invention and ComparativeExamples 1 and 2 was evaluated in accordance with the following methodand criteria by means of a modified machine obtained by mounting a UVirradiation lamp (having a maximum radiation intensity of 1,500 mW/cm²in the range of 300 to 400 nm) on a bubble jet printer PIXUS 550i(manufactured by Canon Inc.) having an on-demand type multi-recordinghead for discharging ink by applying thermal energy corresponding to arecording signal to the ink so that the lamp would be adjacent to therecording head portion as show in FIGURE. Table 2 summarizes the resultsof the evaluation.

(2-1) Fixability

A 12-point letter pattern was printed on PPC paper (manufactured byCanon Inc.) with a predetermined ink by means of the modified machineand irradiated with ultraviolet light to prepare a sample. 10 secondsafter the printing, Silbon paper (lens-cleaning paper) was put on thepaper and was drawn in a state where a weight with a load of 40 g/cm²was mounted on a recorded surface. Whether or not contamination occurredat this time on a non-printed portion (white-background portion) of therecording medium and the Silbon paper owing to friction of a printedportion was visually observed and evaluated according to the followingcriteria.

A: Almost no contaminated part due to friction is observed.B: A contaminated part due to friction is slightly observed.C: A contaminated part due to friction is remarkable.

(2-2) Marker Resistance

A 12-point letter pattern was printed on PPC paper (manufactured byCanon Inc.) with a predetermined ink by means of the modified machineand irradiated with ultraviolet light to prepare a sample. 1 minuteafter the printing, a letter portion was marked once with a fluorescentyellow pen (BEAMLINER-S; manufactured by Zebra Co., Ltd.) with anordinary writing pressure, and then the presence or absence ofdisturbance of the letter was visually observed and evaluated accordingto the following criteria.

A: Almost no disturbance of a letter due to a marker occurs.B: Slight disturbance of a letter due to a marker occurs.C: Disturbance of a letter due to a marker is remarkable.

TABLE 2 Evaluation of polymerization performance Com- Com- Ex- Ex-Example parative parative ample 1 ample 2 Example 3 Example 4 Example 5Example 6 Example 7 Example 8 Example 9 10 Example 1 Example 2 Photo-0.8 1.0 1.1 1.3 1.0 1.1 1.2 1.1 1.0 1.0 0.6 0.4 DSC Fixability B A A A AA A A A A B B Marker B A A A A A A A A A C C resistance

(Evaluation of Film Performance)

The active energy ray curable aqueous ink composition according to thepresent invention exhibits high curing performance on account of such areason as described above. However, the flexibility of the inkcomposition may reduce owing to shrinkage on curing so that the inkcomposition may become brittle. Alternatively, the adhesiveness of acured film with respect to a recording medium may reduce so that thefilm is apt to be peeled off. According to the present invention, asmall amount of aqueous solvent component may be added for the purposeof alleviating the shrinkage on curing or promoting an increase in finalcuring rate. The film performance was evaluated by means of a modifiedmachine of a bubble jet printer equipped with a UV irradiation lamp.

(3) Printing Evaluation 2

Each of the active energy ray curable aqueous ink compositions ofExamples 1 to 10 according to the present invention and ComparativeExamples 1 and 2 was evaluated in accordance with the following methodand criteria by means of a modified machine obtained by mounting a UVirradiation lamp (having a maximum radiation intensity of 1,500 mW/cm²in the range of 300 to 400 nm) on a bubble jet printer PIXUS 550i(manufactured by Canon Inc.) having an on-demand type multi-recordinghead for discharging ink by applying thermal energy corresponding to arecording signal to the ink so that the lamp would be adjacent to therecording head portion as show in FIGURE. Table 3 summarizes the resultsof the evaluation.

(3-1) Strength/Flexibility

A 300% solid pattern was printed on a white film, which was made ofpolyethylene terephthalate and had a uniform and clean surface, with apredetermined ink by means of the modified machine and irradiated withultraviolet light to prepare a sample. 10 minutes after the printing,the state of a film at the time of bending of the sample was visuallyobserved and evaluated according to the following criteria.

A: Almost no change occurs in a film.B: A crack slightly develops in a film and the film is slightly peeledoff.C: A crack remarkably develops in a film and the film is remarkablypeeled off.

(3-2) Adhesiveness

A 300% solid pattern was printed on a white film, which was made ofpolyethylene terephthalate and had a uniform and clean surface, with apredetermined ink by means of the modified machine and irradiated withultraviolet light to prepare a sample. 24 hours after the printing, aprinting portion was scratched in a lattice fashion, and the scratcheswere covered with a tape. After that, the tape was peeled off and theratio of adhering and remaining parts (remaining ratio) was visuallyobserved and evaluated according to the following criteria.

AA: A remaining ratio of 90% or more.A: A remaining ration of 80% or more and less than 90%.B: A remaining ratio of 70% or more and less than 80%.C: A remaining ratio of less than 70%.

TABLE 3 Evaluation of film performance Com- Com- Ex- Ex- Exampleparative parative ample 1 ample 2 Example 3 Example 4 Example 5 Example6 Example 7 Example 8 Example 9 10 Example 1 Example 2 Strength/ A A B BA A B B A A A A flexibility Adhe- B AA AA AA AA AA AA AA AA AA A Csiveness

(Evaluation of Image Performance)

The active energy ray curable aqueous ink composition according to thepresent invention exhibits high curing performance as described above.Therefore, the suppression of the blur of the letters or bleeding and ahigh image density are simultaneously achieved even in a state where theink composition has a high surface tension and builds up on the surfacelayer of a recording medium. The image performance was evaluated bymeans of a modified machine of a bubble jet printer equipped with a UVirradiation lamp.

(4) Printing Evaluation 3

Each of the active energy ray curable aqueous ink compositions ofExamples 1 to 10 according to the present invention and ComparativeExamples 1 and 2 was evaluated in accordance with the following methodand criteria by means of a modified machine obtained by mounting a UVirradiation lamp (having a maximum radiation intensity of 1,500 mW/cm²in the range of 300 to 400 nm) on a bubble jet printer PIXUS 550i(manufactured by Canon Inc.) having an on-demand type multi-recordinghead for discharging ink by applying thermal energy corresponding to arecording signal to the ink so that the lamp would be adjacent to therecording head portion as show in FIGURE. Table 4 summarizes the resultsof the evaluation.

(4-1) Image Density

A 100% solid pattern was printed on PPC paper (manufactured by CanonInc.) with a predetermined ink by means of the modified machine andirradiated with ultraviolet light to prepare a sample. 24 hours afterthe printing, the reflection density was measured by means of areflection densitometer Macbeth RD-918 (manufactured by Macbeth). Theobtained result was evaluated according to the following criteria.

A: A reflection density of 1.3 or more.B: A reflection density of 1.0 or more and less than 1.3.C: A reflection density of less than 1.0.

(4-2) Letter Quality

A 16-point letter pattern was printed on PPC paper (manufactured byCanon Inc.) with a predetermined ink by means of the modified machineand exposed to light to prepare a sample. 24 hours after the printing,the sample was visually observed and the blur of the letters wasevaluated according to the following criteria.

A: Almost no blur is observed.B: Blurred letters are slightly observed.C: A large number of blurred letters are observed.

(4-3) Bleeding

For each of Examples 1 to 10 and Comparative Examples 1 and 2, pigmentspecies of the pigment dispersion was changed to Pigment Red 122 andPigment Yellow 138 to prepare other aqueous active energy ray curableink compositions, thereby providing a set of cyan, magenta, and yellowinks.

100% solid portions of cyan, magenta, and yellow were printed on PPCpaper (manufactured by Canon Inc.) with a predetermined ink set by meansof the modified machine so as to be adjacent to one another to prepare asample. 24 hours after the printing, the degree of bleeding at aboundary between these colors was visually observed and evaluatedaccording to the following criteria.

A: Almost no bleeding is observed.B: Bleeding is slightly observed but is acceptable.C: Bleeding occurs to such an extent that a boundary between colors isunclear.

TABLE 4 Evaluation of image performance Com- Com- Ex- Ex- Exampleparative parative ample 1 ample 2 Example 3 Example 4 Example 5 Example6 Example 7 Example 8 Example 9 10 Example 1 Example 2 Image B A A A A AA A A A B C density Letter A A A A A A A A A A B B quality Bleeding B AA A A A A A A A C B

This application claims priority from Japanese Patent Application No.2004-092371 filed Mar. 26, 2004, which is hereby incorporated byreference herein.

1. An active energy ray curable aqueous ink composition comprising atleast water, a polymerizable substance which undergoes radicalpolymerization by an active energy ray, and a polymerization initiatorwhich generates radicals by an active energy ray, wherein thepolymerizable substance is a water-soluble compound having 2 to 6(meth)acrylic groups, and the polymerization initiator is awater-soluble acylphosphine oxide compound, wherein the polymerizablesubstance is a water-soluble (meth)acrylate compound represented by thefollowing general formula (1):

wherein Z represents a residue of a polyol or an epoxy ester of a polyolhaving (x1+y1) hydroxyl groups, x1 represents an integer of 0 to 2, y1represents an integer of 2 to 6, and x1+y1 represents an integer of 2 to6, R₁ each independently represent a hydrogen atom or a methyl group,and R₂ each independently represent an atomic group A represented by thefollowing formula:

wherein R₃ each independently represent a hydrogen atom or a methylgroup, m1 represents an integer of 1 to 5, and n1 represents 0 or 1, andwherein the polymerization initiator is a nonionic or anionicwater-soluble acylphosphine oxide compound represented by the followinggeneral formula (2):

wherein R₄ represents an alkyl group or a phenyl group, R₅ represents analkyloxy group or a phenyl group, and R₆ represents an atomic group Brepresented by the following formula:

wherein R₇ represents —[CH₂]_(x2)— (x2 represents 0 or 1) or a phenylenegroup, m2 represents an integer of 0 to 10, n2 represents 0 or 1, and R₈represents a hydrogen atom, a sulfonic group, a carboxyl group, ahydroxyl group, or a salt thereof, wherein the content of the water is40 to 90% by mass with respect to the total amount of the active energyray curable aqueous ink composition, and wherein the active energy raycurable aqueous ink composition is used for inkjet recording. 2-3.(canceled)
 4. An active energy ray curable aqueous ink compositionaccording to claim 1, further comprising a coloring material.
 5. Anactive energy ray curable aqueous ink composition according to claim 1,further comprising as another polymerization initiator at least onecompound selected from the group consisting of compounds represented bythe following general formulae (3) to (5):

wherein m3 represents an integer of 1 or more, n3 represents an integerof 0 or more, and m3+n3 represents an integer of 1 to 8;

wherein R₁₂ and R₁₃ each independently represent a hydrogen atom or analkyl group, and m4 represents an integer of 5 to 10;

wherein R₁₂ and R₁₃ each independently represent a hydrogen atom or analkyl group, R₁₄ represents —[CH₂]_(x3)— (x3 represents 0 or 1),—O—[CH₂]_(y3)— (y3 represents 1 or 2), or a substituted or unsubstitutedphenylene group, and M represents a hydrogen atom or an alkali metal. 6.An active energy ray curable aqueous ink composition according to claim1, further comprising at least one solvent selected from the groupconsisting of glycerin, ethylene glycol, and diethylene glycol. 7-9.(canceled)
 10. An inkjet recording method comprising the steps of:applying the active energy ray curable aqueous ink composition accordingto claim 1 to a recording medium by using an inkjet system; and curingthe active energy ray curable aqueous ink composition by irradiating therecording medium applied with the active energy ray curable aqueous inkcomposition with an active energy ray.
 11. An inkjet recording methodaccording to claim 10, wherein the active energy ray curable aqueous inkcomposition is applied on the recording medium by applying thermalenergy to the active energy ray curable aqueous ink composition.
 12. Anink cartridge comprising an ink container containing the active energyray curable aqueous ink composition according to claim
 1. 13. Arecording unit comprising an ink container containing the active energyray curable aqueous ink composition according to claim 1, and arecording head for discharging the active energy ray curable aqueous inkcomposition.
 14. An inkjet recording apparatus comprising: means forapplying the active energy ray curable aqueous ink composition accordingto claim 1 to a recording medium; and means for irradiating the activeenergy ray curable aqueous ink composition applied on the recordingmedium with an active energy ray.
 15. An inkjet recording apparatusaccording to claim 14, wherein the means for irradiating the activeenergy ray comprises an ultraviolet irradiation lamp having a lightsource of a wavelength of 450 nm or less, and the ultravioletirradiation lamp has an intensity of an ultraviolet region in a range of500 to 5,000 mW/cm².
 16. An active energy ray curable aqueous inkcomposition according to claim 1, wherein the polymerizable substance iscontained in the ink in a content of 1% to 30% by mass with respect tothe total amount of the ink.
 17. An active energy ray curable aqueousink composition according to claim 16, wherein the polymerizationinitiator is contained in the ink in a content of 0.1% to 7% by masswith respect to the total amount of the ink.